FIG. 1 illustrates schematically a simplified, partially cut-out silicon drift detector (SDD), which is an example of a semiconductor radiation detector used to detect electromagnetic radiation, particularly X-rays. A bulk layer 101 of semiconductor material receives and absorbs the radiation, which causes free charge carriers to appear. One surface of the bulk layer 101 comprises an arrangement of concentric p-type implantation rings, of which ring 102 is shown as an example. The concentric rings are arranged to have electric potentials of gradually increasing absolute value, so that if the potential at the center of the SDD is only some volts, the outermost ring may have a potential of e.g. −150 V. The number of rings is overly simplified in FIG. 1; in a real-life detector there may be dozens of rings.
Together with a cathode layer 103 on the opposite surface of the bulk layer, the concentric rings create, within the bulk layer, an internal electric field that drives the radiation-induced electrons towards the center of the SDD. At or close to the center, an anode is located for collecting the radiation-induced electrons. The SDD of FIG. 1 comprises an integrated field-effect transistor (FET), the electrodes of which are shown as implantations 104, 105, and 106. The innermost implantation ring, i.e. the one closest to the FET, is the anode, from which a connection 107 is made to the gate of the FET. Alternative structures are known, in which the anode is at the very center of the SDD, and an external FET is coupled to the anode for example by bonding a separate FET chip to appropriate parts of the SDD chip.
A circular SDD with the anode and FET at or close to the center of the SDD chip has the inherent disadvantage that some of the measured radiation will hit the FET, which may disturb its operation and cause radiation damage to the crystalline material from which the FET is made. In a structure like that in FIG. 1, the FET will also reserve some active surface area. As an alternative, the so-called droplet-formed detector, also known as SD3 or SDDD (Silicon Drift Detector Droplet) has been proposed. FIG. 2 illustrates schematically the surface of a droplet-formed detector, again with the number and relative size of the structural elements deliberately distorted in favor of graphical clarity. The implantation rings, the stepwise increasing potential of which create the electric field, are asymmetric so that their arched form is relatively wide on one side (on the left in FIG. 2) but narrow and pointed on the other (on the right in FIG. 2). The outermost implantation ring used for this purpose is shown as 201.
The anode region is generally shown as 202 and it may comprise conductive patches (like in FIG. 2) for bonding an external FET thereto, and/or implantations that at least partly constitute an integrated detection and amplification element such as a FET. The asymmetric form of the implantation rings brings the anode region 202 out of the active area of the detector, so it is much less exposed to radiation than in the structure of FIG. 1, and also does not cause any dead zone in detection.
A problem that may occur in drift detectors is the mixing of surface-generated charge carriers with the signal charge. A prior art publication W. Chen et al.: “Large Area Cylindrical Silicon Drift Detector”, IEEE Transactions on Nuclear Science, vol. 39, no. 4, pp. 619-628, 1992, discusses the problem. Chen suggests that the presence of fixed positive charge in an oxide layer on the surface of the detector chip, together with a “river” of radially oriented gaps in the electrode rings, may hold the surface-generated electrons close to the surface and guide them towards the center of the detector, where they are collected through a dedicated electrode. A drawback of Chen's solution is that it requires careful control of the gaps in the electrode rings. It also means that the amount and distribution of the so-called “oxide charge” must be selected in a specific way that may not be optimal from other viewpoints of designing the detector.